Quantum cascade lasers (QCLs) of the type currently known in the art are described, among other places, in U.S. Pat. Nos. 7,903,704; 8,014,430; and 8,121,164,
Driven by a strong demand for a number of commercial and defense applications, research on midwave infrared (MWIR) QCLs emitting in the first atmospheric window (3.5-4.8 μm) have resulted in a significant progress in laser performance over the last several years. (See [Ref. 1] and [Ref 2].) However, since room temperature QCL characteristics could not be fully described by practical models that would not rely on computation-intensive numerical simulations, MWIR. QCL development was mostly guided by general principles, without a systematic analysis of relative contribution of different laser design parameters to overall laser performance. This likely will hinder further progress in laser performance.
While some success has been achieved in calculating threshold current density and its temperature dependence (see [Ref 3]), there is still a significant discrepancy between theoretical and experimental data for slope efficiency of MWIR QCLs.
In a simple model based on the rate equations, slope efficiency can be presented in the following form:
                                          ⅆ            P                                ⅆ            I                          ≈                                            h              ⁢                                                          ⁢              ϑ                        q                    ⁢                      N            s                    ⁢                                    α              m                                                      α                m                            +                              α                w                                              ⁢                      1                          1              +                                                τ                  3                                /                                  τ                  4                                                              ⁢                      η            i                                              (                  Equation          ⁢                                          ⁢          1                )            where NS is the number of cascade stages, αm are the mirror losses, αw are the waveguide losses, τ4 is the upper laser level lifetime, τ3 is the lower laser level lifetime, and ηi is the injection efficiency, which is usually determined by fitting the results of Equation 1 to experimental data. Injection efficiency for MWIR QCLs is typically reported to be in the range of 50% to 60%. (See [Ref, 4] and [Ref. 5].)
The root cause of the problem why simple models do not adequately describe room temperature laser characteristics is that the injection efficiency term is a function of carrier leakage from the upper laser level that is very difficult to fully account for. As a consequence, unintentional changes in injection efficiency often mask targeted changes in laser design. The best approach to study this term would be first designing a structure with nearly ideal injection efficiency and then modifing the structure by changing, for example, band offset to study corresponding changes in injection efficiency in a controllable manner.
Large laser transition energy for MWIR QCLs leads to a high position of the upper laser level, close to the top of the Γ-valley barriers and bottom of indirect-valley quantum wells. As a consequence, it is difficult to entirely suppress these leakage paths in MWIR QCLs. in addition, it is difficult to evaluate individual contributions of the two types of carrier leakage, i.e. leakage through continuum and indirect states.
The situation is more favorable in the case of longwave infrared (MIR) QCLs emitting in the second atmospheric window (8-12 μm). Since laser transition is much smaller, it is easier to confine carriers on the upper laser level.
LWIR QCLs are traditionally designed using lattice matched AlInAs/InGaAs composition that has a relatively small band offset of 520 meV. For emission wavelength of ˜9 μm, this band offset results in ˜250 meV energy spacing between the upper laser level and the continuum states located above the barriers, similar to that of MWIR QCLs. Therefore, the band offset of the lattice matched composition is not sufficient for taking full advantage of smaller transition energy of LWIR QCLs.
The main reason for using the lattice matched composition is that linewidth of the laser transition is expected to increase with increase in band offset, i.e. with increase in strain, which, turn, reduces material . However, we experimentally showed recently that highly strained QCL designs can have line width similar to that of designs based on significantly to lower strain composition. (See [Ref. 1].) Employment of high strain to LWIR QCL design therefore presents a promising way of improving laser performance and studying carrier leakage in QCL structures.